The present invention relates to a method for producing asymmetric radial polymers. More particularly, this invention relates to a method for producing asymmetric radial polymers which have, on average, two arms which are composed of a first polymer type and two arms which are composed of a second polymer type.
Heretofore, several methods have been proposed for preparing asymmetric radial polymers. As is well known in the prior art, radial polymers comprise three or more arms extending outwardly from a nucleus. The asymmetric radial polymers contain arms of at least two different polymers, which polymers may vary as to chemical composition, structure, and/or molecular weight. A principal difference in the methods frequently used to prepare asymmetric radial polymers resides in the selection of a coupling agent which forms the nucleus of the radial polymer. Multifunctional coupling agents such as silicon tetrachloride have been used to form asymmetric radial polymers having three or four arms. Star-shaped radial polymers having many more arms have been formed using a poly alkenyl aromatic compound, such as divinyl benzene, as the coupling agent as described in Canadian Patent 716,645.
Before 1988, such asymmetric radial polymers were made by forming a blend of the different polymeric arms in the desired ratio and then adding the coupling agent to couple the arms to the coupling agent. Those methods resulted in a product having, on average, the desired number of each kind of arm in the asymmetric polymer. The problem associated with producing asymmetric polymers in that manner is that the product obtained is actually a statistical distribution of all possible products, ranging from that having all polymer arms of a first polymer type to that having all polymer arms of a second polymer type.
U.S. Pat. No. 5,212,249 discloses a two-reactor process for producing asymmetric radial polymers which increases the amount of polymer produced having the desired composition of arms. The process involves separately polymerizing the monomers to create separately the two different types of arms (it was not thought possible to polymerize the arms in the same reactor and still achieve a product which is not a statistical blend). Then one of the polymeric arms is coupled to the coupling agent and when that coupling reaction is complete, the second set of polymer arms is coupled to the coupling agent. This maximizes production of the desired species of asymmetric radial block copolymer.
The two-reactor process described above is very advantageous and produces polymers which have very good properties and are useful in adhesive compositions and for a wide variety of other uses. However, when a four-arm asymmetric radial polymer containing two different arm types in a 1:1 ratio is desired, the two-reactor process does have the disadvantage that it still produces polymers which are a blend of structures. While the polymer will have, on average, two polymer arms of each type the actual product will be a blend of asymmetric radial polymers containing three polymer arms of the first type and one polymer arm of the second type, two polymer arms of each type, and one polymer arm of the first type and three polymer arms of the second type. These polymeric components will behave differently in their strength and flow properties and therefore the properties of the final product will differ according to the amount of each polymer component present. For example, a component containing three polystyrene-polydiene copolymer arms and one homopolydiene arm will be of much higher viscosity than the desired component with two polystyrene-polydiene copolymer arms and two homopolydiene arms. A component containing three homopolydiene arms and one polystyrene-polydiene arm will be of much lower strength than the desired component with two polystyrene-polydiene copolymer arms and two homopolydiene arms. Further, the most commonly used radial polymer coupling agent, silicon tetrachloride, is unfavorable towards polydiene hydrogenation for certain hydrogenation catalyst systems. For instance, high levels of chloride ion (produced when a living polymer chain displaces a chloride of silicon tetrachloride) have deleterious effects on certain hydrogenation catalysts, and hydrogenation catalysts which must be extracted with aqueous acid which produces a labile chloride ion during catalyst removal which promotes stress corrosion cracking in most metal vessels.
Therefore, it would be very advantageous to have a method which is capable of maximizing the amount of the desired four armed polymer with two copolymer arms and two homopolymer arms which is produced and which does not require the use of silicon tetrachloride as the coupling agent. It would be even more advantageous to have a coupling agent which gives a higher yield of the desired species than silicon tetrachloride does. The present invention provides such a process and produces such a polymer.